In the Aplysia model system, a researcher has the unique opportunity to study behavior at both cellular and organismic levels. Much remains to be learned about the regulation of sensory transmission during different phases of feeding. B21 is a known mechanoafferent sensory neuron with connections to the motor neuron B8. It has been proposed that transmission of B21 afferent input to B8 is regulated, i.e., gated during feeding motor programs. Proposed experiments will test a hypothesis that postulates that regulation of spike propagation at least partially accounts for gating. This hypothesis is based on the observation that when B21 is stimulated peripherally, an attenuation of spikes is observed in the soma and lateral processes. When B21 is stimulated via central depolarization, spikes are propagated along the lateral process, firing B8. Proposed experiments involve using confocal microscopy to identify sites of contact between B8 and B21 to affirm that B21 synapses with B8 at points distal to sites of attenuation. This will support the idea that failure of spike propagation in B21 is important to its gating activity to B8. In addition, contact sites will be verified using electrophysiology and lesion experiments. In other experiments I will establish the physiological relevance of results obtained in mechanistic studies. Namely, I will determine whether gating occurs during feeding motor programs and if so whether spike attenuation is important in this context, i.e., whether spikes in B21's lateral process are attenuated. In summary, my experiments will give insight into mechanisms used to regulate sensori-motor transmission in the CNS under physiological conditions.